Transvection is the phenomenon whereby DNA on one chromosome can interact with allelic DNA on the homologous chromosome to control the expression of a gene. We have identified a fragment of engrailed DNA which apparently mediates this phenomenon. This fragment of engrailed DNA can "silence" (i.e. turn off or down) the expression of a linked marker gene. This "silencing" of gene expression occurs only when the engrailed fragment is present in two copies. These two copies can be present either at a similar place on homologous chromosomes or by duplication of the engrailed fragment on the same chromosome. Gene duplication can occur by unequal crossing over or during repair after DNA damage. Our data suggests that gene duplication may in some instances lead to a decrease in gene expression due to the duplication of a "silencing" element. The "silencing" of gene expression could play a role in oncogenesis by the "silencing" of a gene encoding a tumor suppressor gene. We are interested in exploring the mechanism of gene silencing.